Since Robey U.S. Pat. No. 4,023,506, it has been known that an artificial mat made of fibers can be used to enhance the stability of a natural turf surface, by providing enhanced holding power for the root system of the natural grass. Thereafter, U.S. Pat. Nos. 4,044,179 and 4,337,283, issued to Haas, Jr., disclosed a surface comprising an artificial mat with upright fibers, with sand filled onto the mat to provide lateral support for the fibers. The primary purpose of the inventions disclosed in these patents was to provide a stable maintenance free surface for a variety of uses, such as a landing area for golf ball driving ranges, or even golf courses, but the primary objective did not include the growing of natural turf through the mat.
Thereafter, European patent applications EP 0 174 755 A1 and EP 0 403 008 A1 disclosed the combination of an artificial mat for stabilizing a natural turf which extends through the mat, wherein the mat includes a horizontal backing and upright fibers supported laterally by a fill layer, such as sand. More recently, U.S. Pat. Nos. 5,489,317 and 5,586,408, issued to Bergevin, disclosed a structure very similar to the other structures, but with the specific feature of the sand fill layer terminating at the same vertical level as the tops of the artificial fibers. As indicated in the Bergevin patents and in the prosecution histories thereof, this relatively thick layer of sand is necessary to uniformly hold the mat down, and also to initiate natural plant growth at a vertical level which is very near to the top ends of the artificial fibers. However, such a substantial sand fill layer atop the artificial mat and surrounding the artificial fibers forms a tight, compacted uniform playing surface. As a result, the playing surface becomes slick and hard once the turf begins to wear. The Bergevin patents also disclose the feature of coating or covering the artificial mat with latex to hold the artificial fibers for 2-3 weeks until the turf is established and roots have grown through the mat.
Another shortcoming of turf systems utilizing an artificial mat such as those disclosed in the Bergevin patents is the interface between adjacent mat sections. When such systems are installed on site at the athletic field, the artificial mats may shift or become exposed in response to player movement which is very detrimental. Sod pieces which become dislodged and pushed around by player movement or divots in the case of a golf course turf are undesirable for athletic fields. Differential shifting or vertical movement of one artificial mat section relative to another also commonly results from rolling the turf, which is a common turf maintenance practice. If the roller does not span the juncture between adjacent mats then the rolled mat often shifts downwardly thereby creating a discontinuity or unevenness in the turf surface and a potential tripping hazard or other undesirable condition. One measure which has been attempted to counteract this differential shifting of the artificial mats is the placement of a heavy mat, such as a geo-textile grid or expanded polyethylene mat to underlay and straddle the juncture between the artificial mat sections. However, this adds to the complexity and cost of such turf systems and often does not cure the problem of differential shifting of the artificial mat sections.
These recent developments in combining artificial fibers with natural grass plants have resulted in only limited advances in the stability of natural turf surfaces, particularly but not exclusively for natural turf surfaces used for athletic fields which are constructed of soils high in sand content. The relatively slow market acceptance of these surfaces to date seems to indicate that there is room for improvement, primarily from an agronomic standpoint.
For one thing, it is a commonly understood turf management practice to provide periodic layers of "top dressing," usually sand, on a natural turf athletic surface, such as a ball field or golf course putting green or tee. The periodically applied layers of top dressing provide stability and some degree of firmness to the ground, but more importantly create and then help maintain an environment conducive to healthy plant growth and rapid recovery from use.
Even though most well kept athletic surfaces for sport purposes are maintained in such a way that the grass clippings are collected, rather than simply allowed to disperse on the surface, with natural grass plants, particularly under the intense maintenance regimes common for professional and collegiate facilities, there is considerable production and decomposition of plant and root matter which can quickly build up on the surface. This decomposing plant matter will eventually form a thick organic layer which is undesirable for athletic playing surfaces. If not properly managed, this organic layer has a severely negative effect on drainage, movement of nutrients and pesticides into the root system and playability, including footing, surface speed, wear tolerance, ball roll and bounce, etc. The organic layer can result in a condition known in the industry as "black layer" whereby the organic matter becomes anaerobic due to poor drainage and a loss of capacity for gas exchange all of which severely limit root growth and overall plant health.
Several turf management strategies are nearly universally recommended and implemented to ameliorate these harmful conditions. One such strategy includes the periodic application of a sand top dressing. However, with respect to the surface disclosed in the Bergevin patents, the application of additional layers of top dressing would eventually result in completely burying the upright artificial fiber, so that they would not be able to perform their intended purpose of providing lateral stability for the turf surface. Also, the application of a top dressing would produce a loose sand layer above the fibers, and therefore not be desirable. For that reason, it is applicant's belief that the application of a layer of top dressing to a surface of that type would reduce the stability enhancing benefits of the fibers, while failure to apply a layer of top dressing would have the undesired effect of allowing the build up of a residue of decomposing plant matter. For these reasons, the written turf management guidelines of one patented product specifically emphasizes that applications of sand top dressing should not be practiced because it would negate the beneficial aspects of that product.
Another commonly recommended and practiced turf management strategy involves aerating a natural turf surface. As with providing a layer of top dressing, aerating is periodically performed to decrease the build-up of organic matter on the surface, decrease compaction, keep the surface well drained and to maintain a tight and vigorous natural turf canopy. Typically, aerating is done 4-6 times per year on high performance sports fields by inserting and removing hollow tines which are a one half to three quarters inch in width into the turf. In order to penetrate an entire thatch layer and enter the root zone area of the natural turf it is necessary to aerate to a depth of three inches or greater.
Unfortunately, with the various prior art artificial mats described above, it is impossible to aerate down to the desired depth because doing so is prevented by the horizontal backing which holds the upright fibers. Again, the manufacturer of one such product recognizes this problem and cites in its management manual that aeration should not be implemented. Thus, while the mat may provide one agronomic advantage of increased stability for the natural turf surface, the backing of the mat hinders the performance of the most basic turf management practices, namely aerating, and sand top dressing.
The horizontal backing of the mat also serves as a barrier between the layers above and below the mat. This can adversely affect drainage of the field or subirrigation of the field. This concern arises particularly with respect to subsurface drainage, irrigation and heating systems, and controls therefor, as disclosed in applicant's currently pending PCT application Ser. Nos. PCT/US95/11891 and PCT/US96/02207. In essence, the mat and its resultant organic layer create a zone above the underlying growing medium which acts to create a "perched water table", a term understood by those skilled in the art. By creating a perched water table at the top of the root zone, downward movement of water and nutrients and upward movement of water by capillary action ("wicking") during sub-irrigation and the natural phenomenon of evapo-transpiration are significantly inhibited.
Oftentimes, a stabilized natural turf of this type is initially grown on a sod farm, on top of a plastic layer, until it is ready to be cut and then transported to the ultimate site where it is to be used. Since the transporter will typically pay transportation costs which are directly proportional to the weight of the material being transported, it is desirable to minimize the overall weight of the components which make up the stabilized athletic turf grown at the sod farm. Large layers of sand atop the artificial mat are counter-productive in this regard.
Another problem with turf systems having an artificial mat and grown on sod farms is the harvesting of the turf and the waste associated therewith. When such turf systems are installed on the sod farm, typically 12-15 foot wide mats are rolled out and the sides of adjacent mats are abutted but not connected. They may be overlapped and pinned down with landscape spikes to keep them from sliding around during sanding, and the spikes are then removed at a later time. After being placed and pinned, the area is top dressed and turfed. When it comes time to harvest, ideally one would like to have the sod cut lines (rolls cut 30"-48" wide typically) match the mat widths. This is virtually impossible because it would mean that the sod harvester would have to follow exactly the original edge of the mat which cannot be seen because it is covered with turf. Any deviation from the mat line during harvest, however, leaves a narrow piece of mat between the mat line and cut line. And because there is not sufficient integrity for the roots along the cut line to hold that wedge of mat to the adjoining piece, the entire roll could become wasted, as it is critical to maintain precise roll widths. For this reason, harvesters of such systems typically begin cutting rolls 6"-12" on either side of the mat line, leaving that much of a strip wasted.